Wireless networks are becoming more and more popular. With the popularity of wireless networks increasing, users also demand broader coverage from a wireless network, such as voice, video and data communication support. With the broader demands placed on wireless networks, more complex mechanisms are required for differentiated services. For example, users with higher priorities need to be guaranteed higher bandwidth than lower priority users. Unfortunately, wireless local area network (WLAN) protocols such as IEEE 802.11 protocols are designed for best-effort data communications without quality of service (QoS) support.
The current 802.11 protocol has a Media Access Control (MAC) layer that provides a distributed coordination function (DCF) based on a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. According to the CSMA/CA protocol, packet transmissions occur after two waiting periods. First, a channel is sensed idle for a first waiting period termed a DCF interframe spacing (DIFS) period. The second waiting period is an additional backoff period, which is a random time period.
The MAC layer protocol sets the backoff period according to a contention window. A contention window (CW) is a range of values from which a random backoff period is chosen. Specifically, before transmission, a backoff period is computed by finding a random value in the range from 0 to the CW. The backoff period is then computed using the random value: Backoff=Rand (0, CW) * Tslot. Tslot represents a slot time. The time following an idle DIFS is slotted and transmissions only occur at a beginning of a slot. The backoff period is used to initialize a backoff procedure. If the channel is idle, the timer is decreased. If another transmission is detected, the timer is frozen. Each time the channel is idle for a period longer than the DIFS, the backoff timer is periodically decremented once each slot time. If a transmission attempt is unsuccessful, the CW is doubled until a predetermined maximum for CW is reached. Thus, the CW is used to determine the random backoff period before attempting a packet transmission. In the MAC layer for the IEEE 802.11 protocol, the parameters are set to be identical for all types of traffic. In particular, at the start of a transmission, the initial contention window (CWmin) is set to be 31 for each flow and the QoS Interframe Spacing (QIFS) is equally set to the DIFS period for all users. As a result, each device is treated identically and no service differentiation is available. Because of the lack of service differentiation, the performance of the multimedia types of traffic and any real time traffic with various QoS requirements is unsatisfactory in current WLAN systems. For purposes of this disclosure, if not specified, the contention window denotes the CWmin.